Ghosting (television)
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In television, a ghost is an unwanted image on the screen, appearing superimposed on the desired image. In a more specific sense, a ghost is a replica of the desired image appearing fainter and offset in position with respect to the primary image.
Common causes of ghosts (in the more specific sense) are:
- Mismatched impedance along the communication channel, which causes unwanted reflections.
- Multipath distortion, because radio frequency waves may take paths of different length (by reflecting from buildings, clouds, etc.) to reach the receiver. In addition, RF leaks may allow a signal to enter the set by a different path; this is most common in a large building such as a tower block or hotel where one TV antenna feeds many different rooms, each fitted with a TV aerial socket (this is known as pre-echo). By getting a better antenna or cable system it can be eliminated or mitigated.
Note that ghosts are a problem specific to the video portion of television, largely because it uses AM for transmission. TV is also transmitted on VHF and UHF, which have line-of-sight propagation, and easily reflect off of buildings, mountains, and other objects.
The audio portion uses FM, which has the desirable property that a stronger signal tends to overpower interference from weaker signals due to the capture effect. Even when ghosts are particularly bad in the picture, there may be little audio interference.
Note also that ghosts are specific to analog transmission. Digital television avoids the problems of ghosts altogether, although it has its own artifacts such as block errors. DVB and ISDB use COFDM modulation, which performs well under conditions that produce analog ghosting, while ATSC uses 8VSB, which performs poorly under such conditions.[citation needed]
Co-channel interference is sometimes called ghosting, but it is very different in nature.
Ghosting can also occur in KVM switches, or in VGA video cables, due to poor electromagnetic shielding.
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[edit] Fun experiment/method of investigating ghosting
It is possible to use the TV as a passive radar system. By putting a small yagi antenna on a broomstick, it would be possible to find the direction in which the ghost is strongest. By measuring the width of the screen and then the distance between the main image and the ghost, it is possible to work out the difference in the distance of the two paths between the transmitter and the antenna.
Note that the speed of light is 3×108 m/s.
In the PAL and SECAM systems, the line is 64 microseconds from start to finish. As the first 12 microseconds of the line are taken up by things like the front porch, back porch, horizontal sync, and the colour burst, only 52 microseconds of time exists for each horizontal line.
In NTSC the active line duration is 52.2 microseconds, and the total line length is 63.55 microseconds.
From these facts, it is possible to measure the difference in the path length between the two paths the TV signal takes between the transmitter and the receiver. In PAL, if the TV is 20 cm (8") wide, and the ghost is 5 cm (2") to the right of the main image, then:
t = 64×10−6 × 5/20 seconds = 16×10−6 seconds
The resulting path difference is 4800 meters (4.8km, or 3 miles).
[edit] Pre-echo
If the ghost is seen on the left of the main picture, then it is likely that the problem is pre-echo, which is seen in buildings with very long TV downleads where an RF leakage has allowed the TV signal to enter the tuner by a second route. For instance, plugging in an additional aerial to a TV which already has a communal TV aerial connection (or cable TV) can cause this condition.
